Short Stroke 2.8

[blue72s]

Henry,

Why did you choose 95mm bore? Why not 98mm or whatever to make 3 liter short stroke?

[Henry Schmidt]

Originally posted by blue72s
Henry,
Why did you choose 95mm bore? Why not 98mm or whatever to make 3 liter short stroke?

Why not a 100mm and for that matter 102 ?
This is not Mac Donalds, and you don't have to super size me.
When Porsche took a clean sheet of paper and started to build a super car, the 959, they chose the 95 mm piston and a 67 mm stroke. If I could get the 67 mm crank I'd build that engine too.
I also feel the 95 mm cylinder is more stable (thicker walls) than the 98 mm. We have seen ring seating problems with the 98 that we don't see with the 95 mm.
I guess more is not always better.
Who knows? Could a 60x95 (2.5) be close at hand?
9200 rpm ?
The Benetton 3.5 liter F1 V8 was 50 X 95 mm.

[blue72s]

Thanks.

BTW:

Originally posted by Henry Schmidt
They (Mahle) do make a 10.5 to 1 compression, 3.0 RSR piston that you can order in a cylinder with SC bolt spacing that will work for a 2.8 SS engine. Part # from Andial is AND 10398495
Keep in mind that a 10.5 to 1, 3.0 piston was designed for a 70.4mm crank so when you install it on a 66mm crank you will loose static compression.
I have measured a loss from .5 to .58 reduction in comp. ratio when doing this conversion.
10.5 to 1 will now be as high as 10.0 to 1 and as low as 9.9 to 1.

Isn't 9.9:1 or 10.0:1 enough?

[Henry Schmidt]

10.0 to 1 for some applications is enough and maybe too much. But for my taste 10.5 is the target figure and if you can make the engine work and survive then you win.

[0396]

Henry,


Great write-up.

Thanks for the educaton !

By the way, long over due, thanks for the help at Steets 15 years ago when I blew my 6 motor ( spun a rod - like you suggested)

Good to see your doing well.

[nabilious]

Henry - wouldn't an SC case be the same as the 3.O Carrera or early turbo?? I never realized there was much difference. If they're the same I'd think we could get SC cores cheaper, or maybe I'm way off here. Set me straight!
Nabil

[Porschekid962]

It is my understanding that the 930/52 3.0 turbo case is capable of accepting a 6bolt crank whereas the 3.0 sc case can only take 9 bolt cranks. That being said I dont think there is much other difference between the two. Seeing as 9bolt 66mm cranks are rather rare, I think the 956/962 used them it gets expensive fast.

Correct me if I am wrong please.

[KobaltBlau]

that's right. The main journals are bigger on the 9 bolt cranks so you can't put a 6 bolt crank in an SC or later case, and as PK said the 66mm 9 bolt cranks are pricey/scarce, whereas 66mm 6 bolts are not. Of course if you are building a motor with a 70.4mm or longer stroke, there is no advantage to starting with a small journaled case that I am aware of, you might as well use a cheap SC or later case.

[KobaltBlau]

below is a post from Henry regarding the tradeoffs of journal size.

Originally posted by Henry Schmidt
When talking about journal size there seem to be three factors.
Drag, load distribution and weight.
Small journal, less drag ( small surface area) , reduced load distribution, less weight.
Large journal, more drag ( larger surface area), better load distribution, more weight.
There is also an issue of strength but I believe that strength can be manipulated by corner radii of a given journal.
Given some of this is true:
Small journal with large radii should be, low drag, reduced load distribution (increase clearance, and oil pressure to correct) weigh less for easier rotation.
In the case Porsche engines, drag produces friction (heat) and in an oil cooled engine, heat is bad.

[andrew15]

Henry - Sorry if I missed it but, did you custom order some JE pistons for this engine? I read that the compression is 10.5:1 with the 66mm stroke - did you just order 11:1 pistons for a 70.4mm stroke from JE ?

As well, could I keep using my 40mm Webers if I went with a slightly milder cam? (mostly street car)

Thanks,
Andrew M

[Henry Schmidt]

Yes, they are custom built by JE.

40s on a mild 2.8 would work if you don't plan to rev it. But why build a 2.8SS if you're not going to rev it?

[andrew15]

so, some 46 PMO would be in order then...

[paulweir]

Very nice Henry! Thats my style of Porsche 911 engine, gather up a bunch of factory parts (or factory style parts) and build something cool out of them. I have always liked those magnesium stacks much better than the plastic ones. I never use the oil fed tensioners in my own engines.

[ChrisBennet]

I'm a big fan of "beauty is as beauty does" but that motor is also easy on the eyes. Nice work Henry!
-Chris

[red964]

A good 30 years to age that glass shroud would be nicer.
Is that a new one?

[kenikh]

A bump for those who haven't read this. I just reread it and still love it.

[Turbo_pro]

I just found this old post and thought others might enjoy it.

I love that old school Porsche kit more and more.

[kenikh]

Originally posted by solowerks
I just found this old post and thought others might enjoy it.

I love that old school Porsche kit more and more.

With good reason.

[pieterk]

Originally posted by solowerks
I just found this old post and thought others might enjoy it.

Indeed; great thread; thank you!

I'd be curious to hear anyone's opinion on how this remarkable and beautiful motor might compare to a 2.8S and 2.8 RSR. I've read everything I can find on the forums by Grady, Steve and others about the 2.8S, and it seems like a fantastic option to build on a 2.7 case, with more reliability than the RSR. The RSR seems a little more high strung compared to the 2.8S--more of a pure race engine--but all three sound brilliant, and this is the first I've ever heard of a short stroke 2.8.

In the long term, one of these directions is the one I will probably take, currently driving a 2.7RS that I love.

Beautiful work!

[Henry Schmidt]

The 2.8SS can not be built on a 2.7 core. (easily)
It is built using a 3.0 turbo or 3.0 Carrera case.
The benefit of this case is that uses the 3.0 head configuration.
These heads flow better than the 2.7 head because of improved valve angles and bigger valves.
The reason I prefer the 2.8SS over all other 2.8 configurations is that the 66 mm crank is stronger than 2.7 based cranks and the rods have a better rod length to stroke ratio. This ratio reduces rod angularity and increases piston dwell.
Net result: more power, higher functional RPMs (8500) and less destructive forces produced by the pistons.
Big bore/small stroke engines for a given displacement (all other things being equal) produce more horse power. The is why F1 rules specify displacement and max. bore not stroke. Current F1 rules state 2400 cc and a 98 mm max. bore. If you do the math that's a 39 mm stroke (rough guess)

The reason you haven't heard of the 2.8SS before is because it's " The best engine Porsche never built"

They came close with the 959 (95 x 67) and some 956/962.
These were special because they used 9 bolt racing cranks (rare but available) instead of the 6 bolt crank.